Pellicle bean splitters are known. Currently available pellicle beam splitters are relatively large in size and consist of a nitrocellulose membrane or pellicle stretched over a rigid frame. The frames are often fabricated from metal, such as aluminum.
A light source is directed at the membrane and a known fraction of the optical amplitude is reflected while a majority of the optical amplitude is transmitted through the membrane. Pellicle beam splitters are useful in monitoring the amplitude of the light transmitted through the beam splitter. The known fraction of the optical amplitude that is reflected can be transmitted to a monitor photodiode where a determination can be made as to whether an adjustment to the optical amplitude is necessary.
As noted above, optical beam splitters are relatively large in size and cannot be used in smaller applications such as telecommunication modules and other applications that use semiconductor lasers as the light source. Accordingly, there is a need for a beam splitter that is as effective as a pellicle beam splitter in transmitting a majority of the optical amplitude while reflecting a known fraction of the amplitude for monitoring purposes and that further is small enough for the telecommunication modules and other laser applications.
There is an increasing demand for tunable lasers given the advent of wavelength-division multilplexing (WDM) which has become widespread in fiber optic communication systems. WDM transponders include a laser, a modulator, a receiver and associated electronics. One WDM transponder operates a fixed laser in the near-infrared spectrum at around 1550 nm. A 176 wavelength system uses one laser per wavelength and therefore such a system typically must store a 176 additional WDM transponders as spares to deal with failures. This high inventory requirement contributes to the high cost of these systems.
In response, tunable lasers have been developed. A single tunable laser can serve as a back-up for multiple channels or wavelengths so that fewer WDM transponders need to be stocked for spare part purposes. Tunable lasers can also provide flexibility at multiplexing locations, where wavelengths can be added and dropped from fibers as needed. Accordingly, tunable lasers can help carriers effectively manage wavelengths throughout a fiber optics network.
Two currently available tunable lasers are distributed feedback (DFB) lasers and distributed brag reflector (DBR) lasers. A conventional tunable laser module 10 is illustrated in FIG. 1. In tunable lasers, the output power is most often measured from the front of the laser diode gain chip 12 of the laser 11, and not from a rear facet as is done with non-tunable lasers. The output of the laser diode gain chip 12 is directed through a collimating lens 13 and isolator 14. The optical output then engages the cubicle power tap 15 at an angle of about 45° where a fraction of the light is reflected toward a detector shown at 16 and the remaining output passes through the lens 17 to the fiber 18. The detector 16 and diode gain chip 12 are linked by various circuitry shown at 19 for tuning the laser or laser diode shown at 12.
A cube power tap 15 is typically a solid, coated optical element assembled into a standard beam splitter cube that reflects a small portion of the light and sends it to the detector 16 as discussed above. However, one difficulty with the standard beam splitter cube 15 is that it has many surfaces that can provide stray reflections. Although the amplitude of the stray reflections may be relatively small due to antireflection coatings applied to the surfaces of the cube 15, the presence of the reflected light can interfere with small signals that are typical of servo signal inputs used by the control mechanism 19 and diode gain chip 12 to adjust the wavelength of the laser 12.
As a result, there is a need for an improved power tap device which can eliminate the stray reflective rays.